Alternating current rectifier



April 21, 1954 F. E. SMITH ALTERNATING CURRENT RECTIFIER Original FiledSept. 15, 1948 6 Sheets-Sheet 1 TAPS FOR RE DUCED V LTAGE RECTIFIERSYNCHRONOUS 5: new R m. we 5 m PWc E ma ac. LOAD INVENTOR.

FFfl/V/f A. 5/7/777 April 27, 1954 F. E. SMITH 2,677,033

ALTERNATING CURRENT RECTIFIER Original Filed Sept. 15, 1948 6Sheets-Sheet 2 INVENTOR. FPA/v/K 2.. 6/7/77 F. E. SMITH ALTERNATINGCURRENT RECTIFIER Original Filed Sept. 15, 1948 April 27, 1954 6Sheets-Sheet 4 18 B PHASE 17 c PHASE IN V EN TOR.

F. E. SMITH 2,677,033 ALIERNATING CURRENT RECTIFIER 6 Sheets-Sheet 5April 27, 1954 Original Filed Sept. 15, 1948 April 27, 1954 v sMlTH I2,677,033

I ALTERNATING CURRENT RECTIFIER Original Filed Sept. 15, 1948 v 6Sheets-Sheet 6 Q INVENTOR. fFPH/VK 4'. 677/719 Patented Apr. 27, 1954UNITED STATES PATENT OFFICE Divided and this application February 4,1950, Serial No. 142,505

This case is a divisionof application Serial No. 49,333, filed September15, 1948 for Alternating Current Rectifier, now Patent No. 2,594,594,April 29, 1952.

This invention relates to a system for converting alternating current todirect current. It also relates to a synchronous selector for separatingthe alternating currents which proceed in one direction from those whichproceed in the opposite direction, directing them into the load lines ofa D. C. system. It also relates to induction rectifiers. It also relatesto switches, particularly to rotary switches. It also relates tosynchronous selectors employing sodium switches. It relates to a systemof supplying molten sodium to electric systems employing it and includescircuit breaking sodium feeders. It includes an A. C.-D. C. system, aportion of which is sealed and operated in an atmosphere of nitrogen orof other inert gas. The invention also involves a number of dependentbut important concepts which will be more fully set-forth hereinafter.

Most current supply is of alternating current type. industry for directcurrent;v The most satisfactory manner of supplying this demand is toconvert A. C. to D. C., and a number of converters working upondifferent principles have been constructed to accomplish the'conversicn.There are rotary A. C.-D. C. converters, vacuum tubes, mercury arc andcopper oxide converters, as well as those of a mechanicalrswitch typewherein an attempt is made by switches to direct the crests of thealternating current sine waves into one D. 0. line and the nodes intoanother, the load being connected between the two lines. The use ofmechanical switch apparatus to accomplish this result has been tried inmonophase circuits but has been accompanied by such great difficultiesthat there has been no development along those lines in comparison tovery great development in the other types.

Polyphase circuits and systems are most widely used for the distributionof electric power. It is helpful in thinking of polyphase circuits tokeep in mind that any system having more than one circuit is a polyphasesystem, and that each phase may be considered as a separate current orcircuit. Because of their method of generation, poiyphase alternatingcurrents can be represented on a graph by a number of sine wavescorresponding to the number of phases, the several waves being separatedand, in effect, following each other across the graph The portion ofNevertheless, there are large demands in each wave above the zero lineof the graph is considered to be current proceeding in one direction andthat portion of the wave below the zero line is considered to be currentproceeding in the other direction. The object of converting systems isto rectify this current so that it proceeds only in one direction.Alternating current systems of two to twelve or more phases have beenmade and more or less successfully operated, and it is to be understoodthat the several principles of this invention are applicable to all suchsystems. However, the commonest of all systems being three phase A. 0.,this invention will be particularly described with respect to theconverting of that type of current to D. C.

The several prior art types of converter have all been subject to severehandicaps and to material limitations in their efficiency and theircapacity. For example, some of them produce a fluctuating D. C. currentwhich is less than satisfactory for general use and which has a seriousproblem in making the D. C. current flow, be-

cause the A. C. potential is employed to make h the D. C. current flowand the current in a succeeding phase will not flow until the E. M. F.in the preceding phase has fallen below its own E. M. F. This results ina virtual stoppage of current, as one phase relinquishesandthe'su'cceeding phase takes over. A particular difflculty with priorart rectifying systems lay in achieving full use of the potential of thepolyphaseA. C.

, circuit. In general, little use has been made of the negativepotential of such circuitsgAriOther difficulty with the prior artrectifier systems has been expressed as a poor power factor. Stillanother diificulty has arisen by reason of short circuits from the highto the low potential plus phases when the load on or resistance of theline varies so that the current may short between the high and low plusphases.

In the mechanical switch type of rectifier, of which there are very few,there has been great objection because of the tendency of the switchpoints to are and burn out or at least to deposit upon the face of one,metal drawn from the body of the other and thus to make imperfect theclosing of the points. Sometimes these points when subjected to anoverload will burn out, rev quiring the rebuilding-of the apparatus.

A particular need'of any rectifying system is that it shall satisfytherequirements oi. full and varying degrees of load, with efficiency.

It is an object/of this invention to make a rectifying system,?-eperableat substantial unity power factor, whis't'an improvement over the priorart in at least one and preferably in many or all the respectshereinabove noted. In particular, it is an object of the invention toconstruct a rectifying system in which the flow of current in asucceeding phase will be initiated before the potential of that phaseexceeds the potential of the preceding phase.

Another object o1; the invention is to employ the full positive andnegative potential of a pclyphase circuit, and especially to employ thefull negative potential thereof.

Another object of the invention is to reduce. or to eliminate thepossibility of short circuits from one phase of high potential to anadjacent phase of like sign and of low potential.

Yet another object of the lIlVEIllliOH'lS tonrake a synchronous selectorhaving superior switching gear in which the making and breaking ofcontacts is accomplished Without destruction or wear of the contacts,and which. are consequently capable of remaining in use .for. longer.periods of time than has heretofore been possible.

. Anotherobject of the invention is to reduce the magnitude of thefluctuations in the M. of the D. C. system produced by conversion 01A.(3. current.

Yet another object is to satisfy the requirement that the system shalloperate efficiently with full load, or part load. There are many otherobjects of the invention involved because not only the system itself butthe separate-parts thereof involve inventive concepts, and the nature ofthese'other objects of the invention-will be more fully comprehended as.the description proceeds.

In the preferred form of the invention, the system includes a delta typetransformer.

In the accompanying drawings,

Fig. 1 is a diagrammatic view of adelta-con- .nected, three-phase A.C.-D. C. rectifying system including a synchronousselector and aninduction rectifier in both the plus and minus D. C. lines.

Fig. 2 is a side view largely in elevation-otherwise in section, of asynchronous selector.

Fig. 3 is a section on the line 3-3 of Fig. 2.

Fig. 4 is a vertical section through an induction rectifier.

Fig. 5 is a section on the line 5-5 of Fig. 4.

Fig. 6 is an enlarged section through one turn of the windingof Fig. 4.

Fig. '7 is a vertical sectional view of the circuit breaking sodiumfeeder, taken on line 1-1 of Fig. 8;

Fig. 8 is an elevational view of the said feeder.

Fig. 9 is asection on line 9--9 of Fig. 3. Fig. 10 is a diagram of thedisks showing conductor placement for the correct relation ofthree-phase current.

- lines that issue from the delta connected windings of the transformer.secondary. Between these phase lines A, B and C and the respectivewindings of the transformer are taps such as l2, l3, l4 and 15 of the. Aphase, which may be selectively connected to reduce the voltage of thephases and to give the apparatus a flexibility it would-not otherwisepossess.

The three phase vention .will be better understood after a considerationhas been had of their novel structure.

Referring-first to Figs. 2 and 3, A, B, and C indicate the phase linesdiagrammatically shown in Fig. 1 as they are connected to thesynchronous selector. These lines are electrically connected as shown insectional detail in Fig. 3 to a metal tube-24 which is held in aflexible insulator 25 that is in turn sealed in a boss 26 provided witha conical seat 2'! conforming to alike conical seat on the insulator.The insulator is provided with a ledge 28 upon-which rests a washer 29which is penetrated by bolts 30. :Any degreeof sealing pressure can beapplied to the insulator 25 by means of washer 29. As a furtherprotection against the entry or. escape-oi gases to or from the sealedcasing I6 of the synchronous selector, a packing gland 31 maybe,provided in the end of the insulator. The tube 24 is connected at itsouter end'to a tube 32 through which molten sodium is'carried from acircuit breaker feeder -33.

Molten sodium is a good conductor, but-it must be maintained attemperature inorder to remain molten. Consequently, the line-32 includestwo otherconcentric lines 34. of which tube -34 has a sealed end, and 35an open end which terminates short" of the end of tube 34. The hot oilis forced through the pipe 35 intothetube 34 andv the casing E6 to aposition adjacent to a major-double contact or sodium-switch ,point36,the detailed construction of which is shown in Fig. 11 which should nowbe examined.

Carriedby the tube 24 and-the :insulator The bores 38 within thesesleeves extend annular grooves 41 which are connected bysmall passages62 with the bore 38. The face of the contact member 38 isa contact facethrough which the current entering-through the tube 24, and the moltensodium that it-contains is transmitted'to the metal contacts 43 in therotors of the synchronous selector.

The molten sodium makes its way through the channels provided in theparts first described to the grooves in the'faces of the contacts andform a current-conducting film between the rotating face of. the rotorand the fixed faceof the contact 38. The contact 38 is provided with anannular shoulder Moi thinness equal to that of sleeve. 3'! and-a metalbellows 45 connects the two. This bellows is welded to both parts andforms botha seal and a kind of spring member which permits the-movementof the contact. A

spring 46 between the two contacts 38 urges them outward and intocontact withthe rotors. A facing of insulation H is provided for thecontacts 33 and this serves to-clean the approaching face of the rotor,to prevent drawing out a spark.

The contacts 38 may be-otmetal or theymay equally bemade of ceramic orsome non-conducting material inasmuch as sodium will itself trans theseveral grooves of Fig. but with a single cup which receives the sodiumfrom the core 38' of the contact.

There are many advantages inherent in the novel construction which hasjust been described. The molten sodium forms a film between rotor andfixed contact and reduces wear. If an arc is drawn out between arotating and a fixed contact, the sodium film between the faces willvaporize and protect the contacts and the apparatus against damage. Inrotary switches having solid metal to solid metal contact, the how ofcurrent causes pockets to form in one member and accumulations of metalto be acquired by the corresponding contact, necessitating repairs andreplacement of parts, and shortening the use of the machine. By virtueof the new conceptions involving conducting liquid film contacts thelife of the contacts is greatly increased, and the period of use of themachine without halt for repairs is greatly extended.

A shaft is rotatably mounted in bearings52 and passes through packingglands 5| into the casing. It is screw threaded at 53, 53 for thereception of nuts 54 which serve to position annular grips 55, 56, 51.Grip 55 has an annular web 55' projecting centrally from a sleeve 56"and carries an internally bevelled double flange 55". The grip 55 has anannular web 55 projecting from a sleeve 55" and carries a singleinwardly bevelled flange 55". Grip 5! is like grip 55. The sleeves ofthe grips are received over, and sleeve 56" is keyed to the shaft 50, sothat they rotate with the shaft in assembled position. The grip 55 andgrip 56 held between them a rotor 59 which has a hub 80 with outwardlybevelled faces 5| corresponding to the bevels of the grips. Theconstruction of rotor 52 is like that of rotor 59 and is similarlymounted between grips 56 and 51. By adjusting the nuts 54 the positionof the rotors may be precisely adjusted along the shaft 50. A pluralityof copper conductors 63 extend through the rotors from side to side. Inthe form of the invention being described there are five of theseconductors shown in Fig. 9, equally spaced on each rotor and containedwithin arcs of 18 degrees. The rotors are so positioned on the shaftthat the plus and minus discs connect their respective phasesalternately.

The contacts A+, A- are aligned with A; contacts 13+, 3- with B; and C+,C- with C. The conductors are numbered 63 and the B3.

The rotors 59 and 62 are identical except in the angular position oftheir contacts with respect to each other. They are conveniently made ofglass or ceramic material, for instance, porce lain. They are preferablyground and polished after being positioned on the shaft so that thelateral faces will rotate in a true plane and furnish a minimum offrictional resistance to the fixed contacts.

The shaft 50 is driven through a 5 to 1 reduction gearing 54 which is inturn driven by a synchronous motor 65 which takes its power from thesecondary that supplies the system. It must be understood that thisdescription applies to the form of the invention which is beingparticularly described but that different gear ratios could be employedwith other polyphase systems and other numbers of contacts on the discs.

As the motor is driven, current is fed through phase lines A, B and C tothe three double con- 6 l tacts 31 between the discs. shown in Fig. 3the conductor 63 is connecting the contact 3'! with another contact 66which enters through the side wall. At that moment the A phase isdelivering positive current to the line, and that current is transmittedthrough sodium contact 31, conductor 63, and contact 56, the moltensodium furnishing a perfect make and break at each place. As thepositive potential in phase A approaches zero, the contact 63 moves awayfrom contacts 31 and 66 while a contact in disc 59 approaches contacts31 and 61. As the current reverses in phase A, becoming negative,contact is broken by contacts 63 of the disc 62 and contact is madebetween 3! and 61 by a copper contact disc 59 so that phase A is connected through contact 61 and not through contact 66. Inasmuch as asynchronous motor is employed to drive the discs, this make and breakcan be accomplished with precision. The stators of the synchronous motorcan be rotated a little in advance or in retard to achieve withprecision any timing desired.

The conductor and contacts should be within 4 cycle to remove the dangerof the center contact shorting 63 and 63'.

The discs may be rotated at 720 R. P. M. for sixty cycle three phasecurrent and at 300 R. P. M. for twenty-five cycle three phase current.One complete cycle occurs in 72 degrees. The contact brushes of eachsign or will accordingly be located at 96 degrees from each other, sinceB phase is /3 cycle advanced over A, as C is over B.

From each side of the casing Is there issue three contacts, the threenegative contacts being given the numbers 61, 68, B9 and the three pluscontacts, the numbers 10, H, I2. The construction of these contacts withtheir brushes is similar to the construction of 25, differing in thatthere is no T-shaped member 36, the brush 13 extending directly outwardfrom the tube 14 which is set in the insulator 15. The contacts 61, 68and 89 are connected to negative D. C. current lines 22, 2| and 20 whilecontacts 10, H, and 12 are connected to positive D. C. phase lines I9,l8, ll. Each of the phases delivers its positive current in sequence toits own phase line, and the phase lines 20, 2|, 22 receive the negativeD. C. current in sequence.

At this point it is noted that the lines H, l8, l9 could be directlyattached to a bus bar, and the lines 20, 2!, 22 could be directlyattached to another bus bar, the two bus bars serving to supply D. C.current to a load. So far as is known that has been the proposal ofprior attempts to rectify current by mechanical switching. However, suchsystems are faulty in their operation under different conditions. Forinstance, if the circuit is suddenly opened so that there is no loadwhile phase A has high positive potential and phase B very low positivepotential, a short may occur from phase A to phase B to the internaldestruction of the wiring of the system.

I Other difiiculties arise in such systems because of the relativelypulsating nature of the current delivered, there being a veryconsiderable difference between the E. M. F. at the crest of one phasewave and the E. M. F. of that phase at the moment when the succeedingphase takes over. Consequently, I have incorporated in my system novelconceptions calculated to produce a more even how of D. C. current, toreduce or totally eliminate the possibility of short circuits throughthe several and phases, and to de- In the position "*liver power withaxminimum of loss. Conse- ':quently, in combination with thesynchronous--selector, there is included in my preferred sysntemanew apparatus,called an induction rectifier. novel.

The induction rectifiers are themselves A single rectifier 'on the plusside may i be used in one form but the preferred form of 1 the inventionincludes a rectifier on the negative 1 side also.

.An induction rectifier as employed iin thisspecification and the claimis a circuit employing the magnetic field created by the i currentflowing in one phase to resist counterfiow of. current in other phaseswound with it. When so functioning it permits flow in one d1- E rectiononly.

Referring now to Figs. 4, 12, 5 and 6, the numerals 19, 18, H are the +Aphase, B phase and C phase lines respectively of the rectified current.These wires are enclosed in insulation 80 so they are protected againstshort circuiting to each other.

These wires are laid in parallel and are wound'within a fiat steel wire81 which is laid in one direction and again in a flat steel -wire'82which is laid in .the other direction. These three phase wires are thusencabled in a metallic casing of good flux conducting properties, thewire constitution being particularly chosen with that property in'mind.The cable which is given the generic number 83, is wound about acore: 84which may be a permanent magnet. or simply of soft iron or of magneticsteel.

properties.

The core 84 should have good flux conducting 'Itscharacteristics can bechanged by selection of materials for particular rates of "discharge. Itis joined at its ends by yokes 85,

' Wedges 89 support the'windings of the core 83 within the posts 81, 38,which are, in section, 1 more-or less triangular with curved sides.

I it and I9 is direct current.

The current that flows through the lines l'l, Consequently, as

:each of these lines receives the discharge of its connected phase, acircular .fiux about the line is established,-varying in intensity. Thesteel windings 8| B2 invite that flux notonly to travel i in its own.direction but to include in its. circuit all three phase lines.

Consequently, the flux about thecableis the sum of the fluxes existingabout the individual wires and lacks the variation in intensity which ischaracteristic of 5; the fiux about asingle'wire. The coil 83 conisequently acts as a single winding andenerglzes 'the core M,establishing a flOW of flux, such as "indicated by the arrows, which iscontinuous and substantially no loss. due totheir voltage differentials,only one phase offers substantially no resistance and occasions :In allstandard systems,

carries the current and one the negative "current. This means that onephase must carry the full maximum current, and that there is a shockwhen one phase comes'on and the other That influences power factor as itthrows current and potential out of step and contributes to harmonicswhich are disturbing to telephone and radio transmissions. Inparticular, if thereis a low power factor in the supcu'tting in anotherat a low value.

energy in the core of the induction rectifier supplied by one phase istransferred to the succc'eeding phase so that currentstarts to flow in"it before itstops flowing in the first. phase. Furthermore, theinduction stores energy in the core which resists any counterflow inthe. coil.

The induction rectifier is air-cooled in small units and can beoil-coo1ed within a casing in large units. The steel employed in therectifier may desirably lie between the best silicon transformer steeland permanent magnetic steel. The choice will vary with load conditions.The unit permits a low resistance flow in one:direction and imposes highresistance in the opposite direction. There are two windings so closelyassociated that mutual inductance prevents short circuits. There ishelpful mutual inductance between the current carrying lines within thecoil.

The three windings i1, i8, [9 are connected to a bus bar ill from whichthe load 92 draws its current, and the load is connected through bus bar92 to negative lines 20, 2|, 22 in which is another induction rectifier.The use of two .induction rectifiers, one in the and one in thenegative'line, is a necessary feature of the invention employing a deltaconnection and the full and maximum E. M. F. of the A. C. supply.

In order to maintain the system employing sodium at its highesteihciency, the portions where sodium appears are preferably sealed in anatmosphere of inert gas of which nitrogen is exemplary. Before beingadmitted to the sysn the nitrogen is cleaned of all traces of gases "blereacting with the sodium. The molten a conductor and being fed in slowlymoving s reams to the various brushes,would y the current to undesirableplaces were it not for the employment'of circuit breaking feeders ofnovelconstruction. The method of maintaining the sodium in a moltencondition is also novel. In order to maintain all parts of the apparatusin satisfactory running condition, which is neither hot enough to causeexcessive wear in or damage to the system, nor so cold that it congoalsthe sodium, a heating system is provided which employs hot oil forrelatively fixed parts streams of heated-nitrogen whereverneeded formoving parts. These several portions of the invention will now bedescribed.

Referring again to Fig. 2 and Fig. 3, it is to be considered that thecasing l6 hasa sump H30 which gathers the molten sodium which escapesfrom the brushes and returns it through'a pipe line I!!! to a well 32whichinvolves a container its which molten sodium is maintained atalevel K84. A tube H15 extends downward beneath the level of the sodiumand a screen I06 is attached thereto and filters the sodium which isdrawn from the well by a pump it! driven by motor 508. The sodium isforced by the pump through pipe line we to a manifold I10 which extendsabove the circuit breaking sodium feeders. This manifold has a weir HEwhich maintains the sodium in the manifold at a level, the excess sodiumbeing returned through pipe H2 to the sump in casing it. The manifold H0extends alongside the several feeders one and :3.

- portion of another of which are shown in Fig. 7

at 326, and are connected thereto by channels i2l. The level of thesodium in the manifold H8 is above the bottom of the channel. 42! sothat the sodium fiows inward from the manifold to a cup 122 in the headI23 of the feeder. In the bottom of this cup is a needle valve seat 4with which a needle valve stem 525 cooperates, being adjustable by meansof a set screw 26 operable from outside the feeder. This permits thesodium to fall in drops I21 down to the bottom of the feeder where itgathers in a pool I28 and passes through a passageway I29 into thesodium supply pipe 32 which is connected at its other end to one of thebrushes, for instance, to 24 as shown in Fig. 2. The supply pipe 32 issealed at its end by packing gland I30 through which pass the oil supplypipes 34, 35. These concentric hot oil feed pipes extend to thepositions shown in Fig. 3 so that the sodium is maintained at propertemperature throughout, the length of the pipes. It is to be understoodthat in all cases where heat is applied insulation may be employed, butthe showing of such insulation is omitted in order to maintain thedrawings in as simple a condition as is consistent with an adequatedisclosure. The, oil is circulated through dielectric delivery pipesindicated in Fig. 2 at 34' and 35', in which system there is a pump I3Iwhich maintains sufficient circulation in the oil, and a steam oilheater I32 in the same circuit maintains the oil at temperature. Circuitbreaking feeders deliver sodium to each of the -D. C. brushes, circuitbreaking feeders deliver sodium to each of the +D. C. brushes, andcircuit breaker feeders serve each of the A. C. brushes of thesynchronous selector. Hot oil pipe 34" heats the casing.

The circuit breakers'may be of single wall con struction but in cases oflarge electrical capacity are preferably of double wall construction,each wall I53, 'I54 being a glass or other dielectric cylinder clampedbetween the head I23 and a dielectric base member I56. Clamps I51 bearagainst an offset annulus I58 and against the bottom of the base memberI55 in which the sodium orifice I29 is found. The inner cylindrical faceof the member I56 is tapered at I59, forming a tapered fit with thetapered bottom of the tube I54. The head I23 is attached to aninsulating ring I60 by oifset rods I6I, I62, the spacing of such rodsbeing such that adequate insulating distance is maintained between them.The ring has nuts molded in it, in which screw threaded rods arereceived to hold the parts together.

In order to exclude oxygen from the system, it is maintained in anatmosphere of nitrogen under superatmos-pheric pressure. The nitrogen isdrawn from supply I40 through a tube I4I which extends downward beneaththe level of the sodium in the well I02, bubbling up through the sodium,in which all traces of undesired gas are removed, and passes out fromabove the surface of the sodium through a pipe I42 to the easing I6,from whence it passes by a pipe line I43 to a blower I44 and a steamheater I45 by which it is raised to a temperature suflicient to keepsodium molten. From the heater it passes by pipe line I46 to a pluralityof lines I41 to nozzles I48 which are directed to the faces of the discs59, B2 and maintain them at a temperature which prevents the congealingof the sodium. A nitrogen line I49 extends from the nitrogen heater or.one of its lines to a position above the sodium feeders and is connectedthereto by pipes I50 as shown in Fig. 2 and Fig. '7. A small hole II inthe feeder admits nitrogen from tube I50 to the interior of the breakerfrom whence it is enabled to escape in a minor amount through smallorifices I52. From the orifices I52 the nitrogen is exhausted frombetween the two glasses I53, I54 forming the wall of the feeder and outthrough the opening I55. The size of the orifices I52 and I55 may besmall enough to make 10 the loss of nitrogen negligible and theconstruction of return lines unnecessary.

An advantage of this invention is in the construction of an A. 0-D. C.rectifier or converter system which is superior to the known systemsincluding the copper oxide rectifiers, the Inercury arc rectifiers, andto monophase mechanical switching arrangements of the sort heretoforeattempted.

The combination with a polyphase system of synchronous selector andinduction rectifiers marks a material advance in the construction ofconversion systems. The synchronous selector as described with itssodium contacts is adapted to precise construction and to long,continuous operation without halt for repairs. Almost the entireconstruction of this synchronous selector is new, including the switchesper se and the brushes. The method of employing sodium as a conductor isnovel and has material electrical and mechanical advantages. Theprotection of the sodium from reactive gases is simple and satisfactory,and the means. of delivering it to the point of use, convenient.

The induction rectifier protects the system against shorts andmalfunctioning of the circuits and improves the efficiency ofrectification. The construction of the apparatus is simple in comparisonwith the results accomplished.

One of the notable electrical advances of the present invention is thattwo phases of the A. C. may deliver power simultaneously to the D. 0.lines whereas in previous systems the delivery of on terminated as thedelivery of the next began. This overlap constitutes a material advancein the art.

k In certain places the term polyphase D. C, is used, and although it isunusual,-signifies a direct current substantially uniform in flow and E.M. F. supplied by a polyphase A. C. system in which the full E. M. F. ofthe A. C. system is directed to produce the E. M. F. of the D. C. loadline. Where D. C. phase Wires are referred to, it signifies a system inwhich separate wires carry the separate rectified output of the phasesof a polyphase A. C. system.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that I do not limit myself to the specific embodimentsthereof.

What is claimed is:

1. An electrical contact including a metal conduit terminating in asleeve, a tubular piston in the sleeve having a perforated face and anenlarged head, a metal bellows sealing the joint between head andsleeve, 9. tube within the conduit having a closed end, a tube withinsaid tube having an. open end, pressure means to supply the conduit witha liquid conductor such as sodium, and pressure means to supply thetubes with hot oil.

2. An electrical contact including a conduit terminating in a sleeve, atubular piston in the sleeve having a perforated face, a metal bellowssealing the joint between piston and sleeve, a tube within the conduithaving a closed end, a tube within said tube having an open end,pressure means to supply the conduit with a, liquid conductor such assodium, and pressure means to supply the tubes with heating fluid.

3. An electrical contact including a conduit terminating in a sleeve, atubular contact movable in the sleeve and having a, perforated face,sealing means for the joint between the contact 11 and sleeve, a heatingtube within the conduit, means to supply the conduit with a liquidconductor such as sodium, and means to supply the tube With heatingfluid.

4. An electrical contact including a conduit terminating in a sleeve, atubular contact movable in the sleeve and having a perforated face,sealing means for the joint between contact and sleeve, and a heating"tube within the conduit.

5. An electrical contact including a conduit terminating in a sleeve, atubular contact movable in the sleeve and having a perforated face, anda heating tube within the conduit.

6. An electrical contact including aconduit terminating in a tubularperforated contact, and heating means forthe conduit.

7. An electrical contact including a conduit for a liquid conductorterminating in a perforated contact, means to now a liquid conductorthrough the conduit,and means to move a cooperating if contact into and"out ofcontact with the liquid conductor issuing from t e perforated,contact.

'3. An' eiectncarcentact "including a conduit for a liquid conductorterminating a perforated contact, "and a rotarycohftact member bearingagainst the said perforated face.

9; Ari electrical contact including a conduit for a liquid coin'zluctorterminating in a perforated contact face'fa cooperating contact memberbearing against the saidperforated face, and pressure means to now theliquid conductor through the perforationsbetweenthe contacts.

10. A rotary switch including metal contact members connected'by a filmof liquid sodium.

'11. Ase/itch h'avihg'a rotatable disk. provided with a plurality ofmetal contact members, a fixed metal contact adapted to be engaged bysaid contacts in sequence, and means to maintain a film ofliquid'sodiu'muponthe face of said fixed con- An electrical switch including at leasttwo contacts, means to move one said contact repeatedly into and out ofconductive proximity to thepther said contact, and means to introduce 12and to maintain a film of electrically conductive liquid between saidcontacts whereby to reduce arcing, to enlarge the area of contactbetween the said contacts, and to reduce the resistance of the switch tothe flow of current.

13. An electrical switch. including at least two metal contacts, meanstomove one said contact repeatedly into and out of conductive proximity tothe other said contact, and means to introduce and to maintain a film ofmolten sodium between said contacts whereby to! reduce-arcing, to.enlarge the areaofcontact betweenthesaidcontacts, and to reduce. theresistance ofthe switch to the flow oflcurrent.v

4. An electrical switch including at leastitwo 'metal contacts, means tomove one said contact repeatedly into and out of conductivity Proximityto the other saidxcontact, andmeansto intro: duce. and to maintain afilmv of .molten sodium 7 between said contacts whereby to reducearcing;

to enlarge the areaaof contact i ibetweenh the said contacts, 7 to. 7reduce the resistance-k of the switch to the. flow-i of current, one,said .contactbeing perforated, and said introducing means compris: ing aconduit filled withmolten sodium connected to said perforated contact;

References. C e i he 1 9f t is. Pa en UNITED STATES PATENTS Number Name.Date 895,044 Rothenstein Aug. 4, 19.08 1,042,565 Kroner Oct. 29, 19121,653,823 Pudelko Q--- Dec. 27, 1927 1,804,076 Arutunofi I May 5, 19 1,1,913,522 Thompsonet a1, June :13, 933 2,444,687 Widakowich July 6, l9 48 ates ice Hi'ckernell i Feb. ;2l 1950 FOREIGN; PATENTS: Number CountryDate 8,888 Denmark Sept. 2;,1906 20,984- l Sweden June 9, 1906- 634210Germany; Aug..20, 1936

